Abstract

Most studies on the interactions between dolphins and trawl fisheries have focused on the opportunistic feeding of dolphins on discards at the surface. Little is known about sub-surface associations between dolphins and actively fishing trawl nets. Using underwater video footage recorded inside trawl nets, we evaluated behavioural aspects of interactions between common bottlenose dolphins (Tursiops truncatus) and trawl nets in the Pilbara trawl fishery. The interaction rates were very high; in 85 hours of footage collected from 36 trawls, dolphins were recorded inside trawl nets during 29 (81%) trawls and outside trawl nets in 34 (94%) trawls, and for up to 98% and 99% of the trawl duration, respectively. The proportion of foraging behaviours was significantly higher for dolphins inside the net (54%) than those outside the net (31%), indicating that dolphins were presented with a concentrated food source inside the net. Dolphins observed outside the net spent time ‘trampolining’ and socialising, indicating that they were motivated by several factors to approach and interact with trawl nets. Twenty-nine individuals were identified inside the net, some returning to the net numerous times within each trawl and between different trawls and fishing trips. Our results suggest that these dolphins are highly motivated to interact with trawl nets and that entering nets may be a specialised behaviour exhibited only by a subset of trawler-associated dolphins. We conclude that gear modifications, not spatial or temporal adjustments to fishing effort, have the greatest potential to reduce dolphin bycatch in this fishery.